rotational-cipher: add starter implementation

exercises/rotational-cipher/src/main/java/RotationalCipher.java

+class RotationalCipher {
+
+    RotationalCipher(int shiftKey) {
+        throw new UnsupportedOperationException("Delete this statement and write your own implementation.");
+    }
+
+    String rotate(String data) {
+        throw new UnsupportedOperationException("Delete this statement and write your own implementation.");
+    }
+
+}